phpcr/src/PHPCR/Transaction/UserTransactionInterface.php

You can acquire the transaction manager from a session supporting transactions with \PHPCR\SessionInterface::getTransactionManager()

A transaction is started with begin() and only permanently persisted if commit() is called. If commit() is not called until timeout, the lifetime of your php script or if rollback() is called explicitly, no changes are persisted.

Remember that session changes are never persisted before you call $session->save(). Transactions are only necessary if you want to be able to rollback over more than one save operation.

The usage looks like

   $tm = $session->getTransactionManager();
   $tm->begin();
   //do stuff with the session
   $session->save();
   //do more stuff
   if (problem) {
       $tm->rollback();
   } else {
       $session->save();
       $tm->commit();
   }

A transaction manager might support nested transactions, meaning you can call begin() repeatedly without commit() in between (but have to commit every transaction you started).

Remember that in the context of PHPCR, the rollback operation will only reset the transaction but keep the current session changes. If you want to get rid of them too, use \PHPCR\SessionInterface::refresh()